1. Field of the Invention
The invention relates to a device for fusing toner on a carrier of electrostatic images, the carrier migrating through a fusion zone which is heated by infrared radiation.
2. Description of the Prior Art
After electrostatic images have been developed with toner powder, it is known to fuse the powder images onto the image carrier by means of heat radiation. The image carrier can be paper, a film material, printing plates or a like material, on which the toner powder is heated above its melting point by means of a source of radiant heat and is fused on after cooling.
For this purpose, German Auslegesschrift 1,063,029 has disclosed a fixing device having a series of sources of infrared radiation, which are arranged successively in the running direction of the image carrier and each of which extends transversely to the running direction across the total width of the image carrier. A further device described in this paper for fixing the powder image consists of a single radiator which is accommodated in a reflection casing which forms an image of the heating filament of the radiant source in a focal line within the powder image. In this way, the radiant energy is concentrated onto the powder image and causes the powder to fuse without excessive heating of the image carrier. It is possible here to operate the radiant source in a pulsed manner so that the action of the infrared radiation takes place only during a very short period of time.
The heat-fixing device disclosed in German Offenlegungsschrift 1,797,010 consists of a source of radiant heat, which is surrounded by a reflector which focuses the total energy along a narrow strip transversely to the running direction of the image carrier.
An elongate heat source which is located in the interior of a drum having a shell which is transparent to heat radiation is described in German Offenlegungsschrift 1,816,174. This source of radiant heat is associated with an optical device which collects the heat radiation in an image line on the image carrier in order to warm the particles of toner to sufficient temperature and hence to soften them so that they can be fused onto the image carrier in a downstream pressure gap which is formed by a roller pressing against the drum.
The known fixing devices having sources of radiant heat which extend continuously across the width of the image carrier, have the common feature that the power of the radiators falls towards their ends. This results in a non-uniform surface temperature distribution in the direction transverse to the running direction of the image carrier, and hence the burning-in of the powder image becomes non-uniform. When a fixing device of this type is operated with material continuously passing through, there is the further disadvantage that the sources of radiant heat are always switched on and off for the same length of time, with the result that various parts of the fixing device, in spite of cooling, are gradually heated up, and this leads to different degrees of fusion of the powder images onto the image carrier. This effect is undesirable particularly in the case of printing plates as image carriers, from which a coating still has to be removed after the powder image has been fused on, since a complete removal of coating from the printing plates is no longer ensured if the powder images have been excessively burned in.